Several methods to remove plastic thread forms from injection molds upon completion of the injection molding processes exist. The most common methods are as follows:                1. Unscrewing of the threads using rotating cores actuated by hydraulically operated racks or other mechanisms. The rotating cores are located in fixed locations and eject (push) the molded thread forms from the mold by rotation of the cores and a mechanically actuated, mechanically timed (cam action) stripper plate.        2. Unscrewing the molded part from a stationary (fixed) core through the use of a hydraulically actuated rack rotating a pinion assembly, which rotates “ratchet rings”, for example, a Husky Rotating Ratchet Ring system. Ejection is done by the movement of a pneumatically actuated piston pushing against a stripper plate the forward motion of which is mechanically controlled by an external cam follower.        3. Unscrewing with the use of gear trains actuated by mechanical splines (Hasco system) and ejection by the movement of a mechanically actuated, mechanically timed stripper plate (see U.S. Pat. No. 6,599,115 to Chalcraft et al.).        4. Rotation of the threaded cores with gears actuated by electric or hydraulic gear motors and then ejected by the movement of a mechanically actuated, mechanically timed (cam action) stripper plate.        5. Mechanically stripping of the threaded parts from stationary (non-rotating) threaded cores. A stripper plate actuated by a hydraulic cylinder or the injection molding machine ejection system does this.        6. Unscrewing the rotating cores from the molded thread form with hydraulically actuated racks. The rotating cores move downward into a stationary threaded nut.        
The method most commonly used during the injection molding process employs the use of rotating cores actuated by hydraulically operated racks or other mechanisms as described in Method 1 above. In all but the last method, a stripper plate is used to assist with the removal of the molded parts.
In these molds, the stripper plate needs to rise in time with the thread pitch of the molded part. The purpose of the stripper plate is to maintain pressure against the injection-molded part and to insure a controlled vertical movement off of the rotating threaded core. Anti-rotation “teeth” are normally molded into the bottom of the threaded closure. This is to insure that the closure is held in a fixed location while the threaded core unscrews from the inside of the molded part.
In virtually every case, the upward speed and movement of the stripper plate is controlled by an external camming mechanism that is attached to the same “cross-head” mechanism that moves the racks forward and back. Springs are the most commonly used method to assist the stripper plate back to its closed (home) position.
The external location of this camming mechanism usually requires a mold base that is wider than necessary to accommodate the rotating cores only. This additional mold base width is used to contain guides and supports for this mechanism. It is a system that requires components such as the pitch cam, the rapid rise cam, the lifter cam, rollers; guides, wear plates and the cam bar used to contain many of these components.
The injection molding sequence that uses the conventional rack and external camming mechanism is as follows:                1. With stationary and movable halves of the injection mold mounted to the stationary and movable platens of the molding machine, with the machine in the open position, and with the crosshead assembly which holds the gear racks in the up (home) position.        2. The movable half of the mold closes with the stationary half of the mold.        3. Melted plastic material is injected into the cavities of the closed mold.        4. The material is cooled during a timed holding period.        5. When this period ends, the movable half of the mold separates from the stationary half of the mold.        6. A hydraulic cylinder actuates the crosshead assembly to which one or more gear racks are attached. These racks rotate cores that have a thread form machined onto them. The rotating threaded cores will also have a spur gear tooth form on one diameter and this will engage with the rack. The initial forward motion of the cylinder is used to rotate the threaded cores in such a direction so as to unscrew the molded thread from the rotating core. This rotation is normally in a clock-wise (CW) direction.        7. While the threaded cores are rotating, a stripper plate is mechanically lifted in time with the pitch of the molded thread form in order to assist with the ejection of the molded part. The timed movement of the stripper plate is accomplished by the external camming mechanism.        8. Once the part has been ejected from the mold, the hydraulic cylinder, external camming actions, stripper plate, crosshead mechanism and rack must return to their original position in order to be in place for the next cycle. Once all components reach the home position, the mold will once again close and be ready for the next molding cycle.        
The unscrewing system as described adds time to the molding cycle, reduces production efficiencies, contributes to component wear and increases the maintenance costs involved with keeping a mold in operating condition. But, at present this is “state of the art” technology. Visit any molding room and you will find this type of mold in operation wherever closures are being produced.
Here are a few examples of wasted time, wasted motion and wasted money.                1. As the mold opens, the hydraulic cylinder, crosshead assembly, racks, rotating cores and external camming mechanism all move in a forward motion (under load) to unscrew the molded part. Once the part has been ejected, the movement of the hydraulic cylinder is reversed and it returns (without load) to the home position. During this operation, all mold components (racks, rotating cores, camming mechanism, etc.) are also returned to their original position. The reverse movement without actual work being produced is wasted motion and wasted time.        2. The external camming mechanism also includes a “rapid rise” cam segment. Once the unscrewing of the threaded closure is complete, the rack continues its' forward movement along this “rapid rise” cam. The purpose of this cam is to quickly raise the stripper plate at an accelerated speed to shake off or separate and/or eject any molded parts that may want to stick to the stripper plate and anti-rotation teeth. This distance of travel is added to the distance required to unscrew the closure. Since unnecessary distance relates to unnecessary time and unnecessary component wear, this system contributes to lost time and increased costs.        3. Due to the time required to return the cylinder and all components to their original position valuable production time is lost before another molding cycle can begin.        4. Due to the unnecessary distance all components must travel while the cylinder is being returned to its original position, wear on bearings, bearing surfaces, dynamic seals and all moving components is doubled. This requires more maintenance cycles per year. These maintenance cycles take the mold out of production (perhaps for several days at a time) and have a dramatic impact on production output.        5. Due to the use of the external camming mechanism and all related hardware, the size of the mold is increased. This increase in size may adversely affect the overall size of the mold to a point that it will require a larger molding machine. This usually means increased pricing because of higher machine rates for larger molding machines.        